1. Field of the Invention
The present invention relates generally to a gas turbine engine, and more specifically to an air cooled turbine airfoil with trailing edge cooling.
2. Description of the Related Art including information disclosed under 37 CFR 1.97 and 1.98
A gas turbine engine includes a turbine section with multiple rows or stages or stator vanes interposed with rotor blades that react with a hot gas flow passing through the turbine. One process for increasing an efficiency of the turbine, and thus the engine, is to pass a higher temperature gas flow into the turbine (referred to at the turbine inlet temperature, or TIT). However, exposing the turbine airfoils to higher temperatures requires improved materials or better cooling.
The trailing edge region of an airfoil, such as a turbine rotor blade, is exposed to some of the highest gas flow temperatures. Also, the trailing edge of the airfoil is thin in order to prevent the flow from separating downstream. FIG. 1 shows a prior art turbine blade with a trailing edge cooling circuit in which pin fins extend across a trailing edge cooling channel. This design for trailing edge cooling requires a relatively thick airfoil at the trailing edge to accommodate the pin fins. In some turbine stage blades, this large trailing edge thickness may induce high blockage and thus reduce the stage performance.
Size and space limitations make the trailing edge region of a gas turbine airfoil one of the most difficult areas to cool. In particular for a high temperature turbine airfoil cooling design, extensive trailing edge cooling is required. FIG. 2 shows another prior art turbine blade which is sued in a first stage of the turbine, and makes use of a pressure side bleed for the airfoil trailing edge cooling. This type of cooling design used to minimize the airfoil trailing edge thickness has been used for airfoil trailing edge cooling for the last 30 years. Problems associated with this type of cooling is the shear mixing between the cooling air and the mainstream hot gas flow as the cooling air exits from the pressure side. The shear mixing of the cooling air with the mainstream flow reduces the cooling effectiveness for the trailing edge overhang and thus induces an over-temperature at the airfoil trailing edge suction side location. Frequently, this over-temperature location becomes the life limiting location for the entire airfoil.
Cooling air exit slots have been used on the pressure side of the trailing edge region of an airfoil, but are long in the chordwise direction of the airfoil. Thin long length discharges the cooling air but does not provide enough cooling for the trailing edge tip. Thus, a metal over-temperature occurs on the trailing edge due to a lack of adequate cooling.